/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive;

import java.util.function.Function;

import reactor.core.publisher.Mono;

import org.springframework.core.MethodParameter;
import org.springframework.core.ResolvableType;
import org.springframework.lang.Nullable;
import org.springframework.ui.Model;
import org.springframework.util.Assert;

/**
 * Represent the result of the invocation of a handler or a handler method.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public class HandlerResult {

    private final Object handler;

    @Nullable
    private final Object returnValue;

    private final ResolvableType returnType;

    private final BindingContext bindingContext;

    @Nullable
    private Function<Throwable, Mono<HandlerResult>> exceptionHandler;


    /**
     * Create a new {@code HandlerResult}.
     *
     * @param handler the handler that handled the request
     * @param returnValue the return value from the handler possibly {@code null}
     * @param returnType the return value type
     */
    public HandlerResult(Object handler, @Nullable Object returnValue, MethodParameter returnType) {
        this(handler, returnValue, returnType, null);
    }

    /**
     * Create a new {@code HandlerResult}.
     *
     * @param handler the handler that handled the request
     * @param returnValue the return value from the handler possibly {@code null}
     * @param returnType the return value type
     * @param context the binding context used for request handling
     */
    public HandlerResult(Object handler, @Nullable Object returnValue, MethodParameter returnType,
            @Nullable BindingContext context) {

        Assert.notNull(handler, "'handler' is required");
        Assert.notNull(returnType, "'returnType' is required");
        this.handler = handler;
        this.returnValue = returnValue;
        this.returnType = ResolvableType.forMethodParameter(returnType);
        this.bindingContext = (context != null ? context : new BindingContext());
    }


    /**
     * Return the handler that handled the request.
     */
    public Object getHandler() {
        return this.handler;
    }

    /**
     * Return the value returned from the handler, if any.
     */
    @Nullable
    public Object getReturnValue() {
        return this.returnValue;
    }

    /**
     * Return the type of the value returned from the handler -- e.g. the return
     * type declared on a controller method's signature. Also see
     * {@link #getReturnTypeSource()} to obtain the underlying
     * {@link MethodParameter} for the return type.
     */
    public ResolvableType getReturnType() {
        return this.returnType;
    }

    /**
     * Return the {@link MethodParameter} from which {@link #getReturnType()
     * returnType} was created.
     */
    public MethodParameter getReturnTypeSource() {
        return (MethodParameter) this.returnType.getSource();
    }

    /**
     * Return the BindingContext used for request handling.
     */
    public BindingContext getBindingContext() {
        return this.bindingContext;
    }

    /**
     * Return the model used for request handling. This is a shortcut for
     * {@code getBindingContext().getModel()}.
     */
    public Model getModel() {
        return this.bindingContext.getModel();
    }

    /**
     * Configure an exception handler that may be used to produce an alternative
     * result when result handling fails. Especially for an async return value
     * errors may occur after the invocation of the handler.
     *
     * @param function the error handler
     * @return the current instance
     */
    public HandlerResult setExceptionHandler(Function<Throwable, Mono<HandlerResult>> function) {
        this.exceptionHandler = function;
        return this;
    }

    /**
     * Whether there is an exception handler.
     */
    public boolean hasExceptionHandler() {
        return (this.exceptionHandler != null);
    }

    /**
     * Apply the exception handler and return the alternative result.
     *
     * @param failure the exception
     * @return the new result or the same error if there is no exception handler
     */
    public Mono<HandlerResult> applyExceptionHandler(Throwable failure) {
        return (this.exceptionHandler != null ? this.exceptionHandler.apply(failure) : Mono.error(failure));
    }

}
